The present invention relates to a motor speed control apparatus for receiving a bit signal from a microcomputer or the like and changing a pulse width of a pulse signal.
In recent years, an electric motor driven pump type power steering system has been developed as a power steering apparatus for a vehicle. A conventional power steering system of this type uses an electric motor as a power source to drive an oil pump. A flow rate of an oil delivered from the oil pump is controlled to allow a driver to operate a steering wheel with an optimal steering force. The speed of the electric motor is controlled in accordance with a vehicle speed and a steering angle, and hence a flow rate of the oil delivered from the oil pump is changed. During low-speed travel requiring a large steering force, the flow rate of the oil delivered from the oil pump is increased. To the contrary, during high-speed travel requiring a small steering force, the delivery amount of oil is decreased, thereby allowing steering with an optimal steering force in a wide vehicle speed range.
In such a conventional electric motor driven pump type power steering system, a pulse width modulation (PWM) scheme generally using a microcomputer is employed as a method of controlling the speed of the electric motor. More specifically, as shown in FIG. 4, information associated with a vehicle speed, a steering angle, and the like is input to and processed by a microcomputer 1, and a pulse signal (PWM signal) having a duty ratio corresponding to the operating condition is generated. The generated PWM signal is supplied to the base of a power transistor 2 through a PWM terminal 1a. The duty ratio of the power transistor 2 can be controlled to change the average voltage applied to an electric motor 3. Therefore, the speed of the electric motor 3 is variably changed to control the flow rate of the oil delivered from an oil pump (not shown). Its typical example is described in U.S. Pat. No. 4,626,994 issued on Dec. 2, 1986. In the system described in this prior art patent, the travel conditions are classified into several modes in accordance with combinations of vehicle speeds and steering angles. In each mode, a combination of the vehicle speed and the pump speed, i.e., a combination of the vehicle speed and the motor speed is changed. For this purpose, the duty ratio of a control signal supplied to the motor is changed to change the motor speed.
In the above method of controlling the electric motor, since the PWM signal supplied to the base of the power transistor 2 is generated in the microcomputer 1, a capacity of a memory for storing a program for generating the PWM signal is excessively increased. Therefore, the microcomputer 1 must have a large capacity. Various operation functions are provided to the microcomputer 1 in addition to the function for generating the PWM signal. There is a fear of interfering other operations due to the time required for generating the PWM signal having a higher priority. As a result, a high-speed, expensive microcomputer must be used.